ABSTRACT
Background
Prolonged postpartum amenorrhea is a sign of secondary infertility. However, there are no reports on postpartum menstrual resumption delay in Japanese women. We conducted an Adjunct Study of the Japan Environment and Children’s Study (JECS) to elucidate the actual state of postpartum menstrual resumption delay in Japan and related factors.
Methods
An Adjunct Study questionnaire was sent to 2,729 mothers who participated in the study of Shinshu Subunit Center in the JECS, and 762 mothers were included in the analysis. The participants were categorized into the normal and delayed groups based on the weeks of postpartum menstrual resumption. To investigate the factors associated with postpartum menstrual resumption delay, a multiple logistic regression analysis was conducted with the weeks of postpartum menstrual resumption as the dependent variable.
Results
The study included 762 women, of which 61 (8.0%) had delayed menstruation, not occurring until 72 weeks postpartum. The multivariate analysis revealed a significant correlation between postpartum menstrual resumption delay and age at delivery, past history of irregular menstruation, history of taking oral contraceptives, breastfeeding at 18 months, and the level of satisfaction with the husband’s participation in childcare.
Conclusion
Postpartum menstruation in Japanese women occurred later than before. In addition to previous findings, another factor was the husband’s satisfaction with his participation in childcare. We should not only focus on the physical aspects of mothers, but also provide midwifery care that proposes and supports family planning suitable for each family.
Keywords: breastfeeding, family planning, maternal age, menstruation, postpartum amenorrhea
The resumption of menstruation after childbirth is undoubtedly an important event for women wishing to have next baby. According to a reliable US report on the resumption of postpartum ovulation,1 ovulation occurred within 12 weeks in 100% of non-breastfeeding women. In breastfeeding women, ovulation occurred by 52 weeks postpartum in 95% and by 62 weeks in 100%. Based on this study, in the field of obstetrics, menstruation resumes at 72 weeks postpartum at the latest. This standard continues to be cited to date in most textbooks and literature, including “Williams Obstetrics,”2 which is the most reliable and highly reviewed textbook. A study on postpartum menstrual resumption in foreign countries reported that the cumulative rate of resumed menstruation in the first 6 months postpartum was about 22.0% to 50.0%.3, 4 Furthermore, the cumulative rate of resumed menstruation among exclusive breastfeeding women in the first 6 months postpartum was 53.2%.5 In Japan, 38.8% at 6 months postpartum6 and 21.2–29.6% at 4–5 months postpartum are reported.7, 8 However, all Japanese reports focused on the resumption of menstruation within 6 months. Thus, to the best knowledge, there is no long-term study that clearly describes postpartum menstrual resumption delay in Japanese women, nor are there any large-scale surveys similar to the present one.
Postpartum amenorrhea periods are longer for breastfeeding women than for non- breastfeeding women because persistently elevated prolactin levels inhibit ovulation.2, 9, 10 Factors other than breastfeeding that are related to early resumption of menstruation include: younger age, multiple parities, early use of family planning, high socio-economic status, exclusive breastfeeding manual removal of placenta,11 short birth interval, few parities, high educational, high socio-economic status and child death.12 However, we have not observed any research reports describing the factors that cause postpartum menstrual resumption delay for longer than 18 months.
In Japan, 18.2% of married couples have received infertility treatment.13 Secondary infertility is defined as “a woman unable to establish a clinical pregnancy but who has previously been diagnosed with a clinical pregnancy.”14 In most instances, postpartum amenorrhea is physiological; however, prolonged amenorrhea may be a symptom of secondary infertility. Therefore, adequate attention should be paid to any prolongation of the postpartum amenorrhea period for the early identification of secondary infertility.
In recent years, in our support of postpartum women, we experienced that it is not rare for menstruation to be absent in women even after 72 weeks postpartum. We assumed that postpartum menstrual resumption in modern Japanese women occurred later than in existing reports. However, to the best of our knowledge, no reports have examined this assumption. Therefore, this study elucidated the current status of postpartum menstrual resumption delay and related risk factors in Japanese women. The results of this study are expected to be useful in midwifery care to provide family planning that is suitable for each family.
MATERIALS AND METHODS
Study design and participants
This study was approved by the Ministry of the Environment and was conducted as an Adjunct Study of the JECS of the Ministry of the Environment. In brief, The JCES is a birth cohort study conducted to determine the effects of the environment on children’s health and development from fetal to early childhood. Pregnant women were recruited from across Japan through 15 Regional Centers between 2011 and 2014. Pregnant women who had difficulty answering the questionnaire in Japanese were excluded. Thus, approximately 100,000 pregnant women have been registered. The survey is still ongoing. The details of the JECS have already been published.15,16,17 Shinshu University was designated as a subunit center conducting the JECS, and 2,729 mothers who gave birth in 8 municipalities throughout Nagano prefecture were enrolled in the study and continued to participate in surveys with their children. For our study, questionnaires were sent to pregnant women in Nagano prefecture registered with JECS at four different time points: 1, 2, 3, and 4 years after delivery. In this study, we used data from 793 participants who responded at least once during 1–3 year postpartum. We excluded 18 participants in whom the weeks of postpartum menstrual resumption were not known, 3 participants with missing data in the JECS, and 10 participants for whom the data of the Adjunct Study and JECS could not be linked. Consequently, we included 762 participants in this study.
Data collection
Information on the timing of postpartum menstrual resumption was obtained from a self-administered questionnaire in the Adjunct Study. In addition, data were obtained from the self-administered questionnaires in the JECS and the Adjunct Studies on the factors that influence postpartum menstrual resumption delay. For the data obtained from the JECS, we used the fixed data set “jets-ta-20190930” released in October 2019.
Analysis items
Information about background factors that might influence postpartum menstrual resumption delay was then selected based on previously published literature and biological plausibility from data in the Adjunct Study and the JECS.2, 9, 10, 18,19,20,21,22,23, 25, 26 We then classified the items into three categories.
Category 1 comprised the sociodemographic and clinical characteristics of participants. We surveyed age at delivery, age at first childbirth, parity, body mass index before pregnancy, highest level of maternal and partner’s education, annual household income during pregnancy, age at first menstruation period, infertility treatment, history of taking oral contraceptives, past history (10 items, including irregular menstruation, Supplemental Table 1), pregnancy complications (16 items, including diabetes, Supplemental Table 1), obstetric complications (17 items, including gestational diabetes, Supplemental Table 1), mode of delivery, gestational age (weeks), Kessler Psychological Distress Scale (K6) score in the first and second/third trimesters and at 1 year postpartum,27 Edinburgh Postnatal Depression Scale score (EPDS) at 1 month and 6 months postpartum,28 the Mother-to-infant Bonding Scale at 1 year postpartum,29,30,31 and current employment status (1 year after delivery). K6 was developed by Kessler et al.27 and is a widely used measure of mental disorder. The Japanese version was validated by Furukawa et al.32 The EPDS was designed to assess pregnancy and postpartum depression by Cox et al.28 The validity and reliability of the Japanese version have been reported by Okano et al.33 The Mother-to-infant Bonding Scale was developed to valuate maternal attachment.29,30,31 Later, Yoshida et al.31 translated it into Japanese and reported its validity and reliability. All items in Category 1 were used in the JECS.
Category 2 included the postpartum childcare environment. We surveyed the following items at 1 year postpartum: level of satisfaction with the husband’s participation in childcare, mother’s frequency of playing with the child, frequency of reading to the child, frequency of taking the child out of the house (to somewhere other than the childcare facility), frequency of meeting other parents with same-aged children, frequency of talking about the child with the husband, availability of a caretaker other than the child’s mother, availability of people from whom the mother can seek advice about childcare, and breastfeeding at 6 months, 1 year, and 18 months postpartum. The level of satisfaction with the husband’s participation in childcare was our own question item; the other items were those used in the JECS survey.
Category 3 comprised information on neonates. We surveyed single or multiple births, gender, Apgar score, pH value of umbilical arterial blood, birth weight, physical anomalies up to 1 month after birth, and disease diagnosed before 1 year of age. All items were used in the JECS survey.
Statistical analysis
“Williams Obstetrics”2 stated that all women resume menstruation at 72 weeks postpartum. Therefore, we analyzed the results by dividing the participants into two groups: the normal group, in which menstruation resumed within 72 weeks, and the delayed group, in which menstruation resumed at 72 weeks or later.
Distribution normality in continuous variables was confirmed using the Shapiro–Wilk test. Data are expressed as non-parametric distribution by median (range). The Mann-Whitney U test was used to examine differences between the delayed and normal groups and to compare the number of weeks of postpartum menstrual resumption between the breastfeeding and non-breastfeeding groups. Fisher’s exact test or chi-square test was performed to compare variables between groups stratified by category. To investigate factors associated with postpartum menstrual resumption delay, we conducted multiple logistic regression analysis with the weeks of postpartum menstrual resumption as the dependent variable (normal group and delayed group). In the three categories, we selected items exhibiting a significant difference on univariate analysis and items in previous research thought to significantly influence postpartum menstrual resumption delay from a medical perspective as independent variables. The items were selected for the following reasons. Age is a significant factor affecting ovarian function and secondary infertility. The average age at delivery of multiparous was found to be higher than primiparous; therefore, we did not select parity influenced by age at delivery. The presence of irregular menstruation and infertility treatment was selected because women with underlying medical conditions may develop ovarian dysfunction and sex hormone disorders. A history of taking oral contraceptives was selected because these medications have an ovulation-preventive effect. Breastfeeding was selected because it is associated with postpartum amenorrhea.2, 9, 10 We hypothesized that postpartum menstrual resumption delay would be influenced not only by pre-pregnancy and pre-partum factors but also by the postpartum childcare environment. Thus, satisfaction with the husband’s participation in childcare was chosen owing to its potential to lead to new findings. We obtained the odds ratio (OR) and 95% confidence interval (CI) using the forced entry method. Spearman’s rank correlation coefficient was used to determine the multicollinearity of the covariates.
All statistical analyses were performed using SPSS Statistics version 28 (IBM, Armonk, NY). A P value of < 0.05 was considered statistically significant.
Ethical considerations
The JECS was conducted in accordance with the Declaration of Helsinki and Japan’s effective laws/regulations and guidelines. The protocol for JECS was approved by the institutional review board for epidemiological studies of the Ministry of the Environment (ethical review number: 100910001) and the ethical review board of all participating institutions. We applied to the JECS steering committee and obtained approval to conduct our study as an Adjunct Study. Subsequently, approval was obtained from the ethical review board of the Shinshu University School of Medicine (approval no. 1842, 3724). All participants provided informed consent to participate in this study.
RESULTS
Postpartum menstrual resumption delay
The cumulative rate of postpartum menstrual resumption in all participants (N = 762) is presented as the solid line (+) in Fig. 1. The cumulative rate of postpartum menstrual resumption was 69.0% at 48 weeks, 92.0% at 72 weeks, and 100% at 132 weeks. Amenorrhea was observed in 8.0% of the participants at 72 weeks postpartum. The median duration of postpartum menstrual resumption was 42 weeks for all participants, 38 weeks for the normal group (n = 701), and 78 weeks for the delayed group (n = 61) (Table 1).
Fig. 1.
Cumulative resumption of menstruation rate according to the number of weeks postpartum.
Table 1. The weeks of resuming postpartum menstruation.
| All | < 72 weeks | ≥ 72 weeks | |
| % (n) | 100 (762) | 92.0 (701) | 8.0 (61) |
| Median weeks (interquartile range) | 42 (22–50) | 38 (22–46) | 78 (74–82) |
The cumulative rate of postpartum menstrual resumption according to breastfeeding and non-breastfeeding women is presented as the solid line in Fig. 2. Based on report,1 estimates of cumulative postpartum menstrual resumption rates from their data are shown as dash-dotted lines in Fig. 2. Menstruation is assumed to occur two weeks after ovulation.
Fig. 2.
Cumulative resuming postpartum menstruation rate according to breastfeeding performed at 6 months (The present study vs. previous study*). Solid line: This study. Excluding cases in which breastfeeding at 6 months after delivery was unknown n = 46. ●Breastfeeding women (n = 658). ΔNon-breastfeeding women (n = 58). Dash-dotted line: Ovulation period in previous study* + 2weeks. ●Breastfeeding women (n = 55). ΔNon-breastfeeding women (n = 22). *Campbell OM, Gray RH. Characteristics and determinants of postpartum ovarian function in women in the United States. Am J Obstet Gynecol. 1993;169:55-60. PMID: 8333476. Altered with permission from Elsevier.
In the 716 participants from whom information concerning breastfeeding at 6 months postpartum was obtained, the median weeks of postpartum menstrual resumptions were 42 (interquartile range; 26–50) weeks for breastfeeding women (n = 658) and 14 (interquartile range; 10–18) weeks for non-breastfeeding women (n = 58) (Table 2). Using Mann-Whitney’s U test, comparison of median values between the two groups revealed significantly earlier postpartum menstrual resumption in non-breastfeeding women (P < 0.01**). The cumulative rate of postpartum menstrual resumption in breastfeeding women was 66.4% by 48 weeks postpartum, 91.2% by 72 weeks, and 100% by 132 weeks, whereas the rate reached 100% in non-breastfeeding women at 68 weeks (Fig. 2). In the normal group (n = 658), the median time of postpartum menstrual resumption was 42 weeks for breastfeeding and 14 weeks for non-breastfeeding women. In the delayed group (n = 58), the median time of postpartum menstrual resumption was 78 weeks for breastfeeding women; there were no non-breastfeeding women (Table 2).
Table 2. The weeks of resuming postpartum menstruation based on breastfeeding at 6 months after delivery.
| All | < 72 weeks | ≥ 72 weeks | ||
| % (n) | 100 (716) | 91.9 (658) | 8.1 (58) | |
| At 6 months after delivery | ||||
| Breastfeeding (n = 658) | 42 (26–50) | < 0.01** | 42 (26–50) | 78 (74–82) |
| Non-breastfeeding (n = 58) | 14 (10–18) | 14 (10–18) | – | |
Data are median (interquartile range) weeks or % (n). We excluded n = 46 in whom breastfeeding at 6 months after delivery was unknown. Calculated using the Mann–Whitney U test. **P < 0.01
Factors related to postpartum menstrual resumption delay
Tables 3, 4, and 5 summarize the participants’ characteristics according to the number of weeks of factors related to postpartum menstrual resumption delay.
Table 3. Sociodemographic and clinical characteristics of participants according to the weeks of resuming postpartum menstruation.
| < 72 weeks n = 701 |
≥ 72 weeks n = 61 |
P | |
| Age at delivery | 31 (28, 35) | 34 (31, 37.5) | < 0.01** |
| Age at first childbirth | 28 (25, 32) | 30 (27.5, 32) | 0.03* |
| Parity | 0.01* | ||
| Primiparous | 43.7 (303) | 27.1 (16) | |
| Multiparous | 56.3 (391) | 72.9 (43) | |
| BMI before pregnancy (kg/m2) | 20.6 (18.9, 22.3) | 19.9 (18.6, 21.5) | 0.09 |
| Highest level of maternal education | |||
| Junior high school / High school | 28.8 (201) | 19.7 (12) | 0.13 |
| Vocational school / Junior college / University / Graduate School | 71.2 (498) | 80.3 (49) | |
| Highest level of partner’s education | 0.35 | ||
| Junior high school / High school | 41.2 (287) | 35.0 (21) | |
| Vocational school / Junior college / University / Graduate School | 58.8 (410) | 65.0 (39) | |
| Annual household income during pregnancy | 0.99 | ||
| < 4,000,000 JPY | 37.8 (253) | 36.8 (21) | |
| 4,000,000~5,990,000 JPY | 34.8 (233) | 35.1 (20) | |
| ≥ 6,000,000 JPY | 27.4 (183) | 28.1 (16) | |
| Age at first menstrual period (year) | 0.26 | ||
| < 15 | 93.3 (637) | 88.7 (47) | |
| ≥ 15 | 6.7 (46) | 11.3 (6) | |
| Infertility treatment | 0.03* | ||
| No | 96.4 (675) | 90.2 (55) | |
| Yes | 3.6 (25) | 9.8 (6) | |
| History of taking oral contraceptives | < 0.01** | ||
| No | 90.4 (628) | 76.3 (45) | |
| Yes | 9.6 (67) | 23.7 (14) | |
| Past history Irregular menstruation |
< 0.01** | ||
| No | 89.1 (620) | 72.9 (43) | |
| Yes | 10.9 (76) | 27.1 (16) | |
| Pregnancy complication Diabetes |
0.10 | ||
| No | 99.3 (696) | 96.7 (59) | |
| Yes | 0.7 (5) | 3.3 (2) | |
| Obstetric complication Gestational diabetes |
< 0.01** | ||
| No | 99.4 (697) | 91.8(56) | |
| Yes | 0.6 (4) | 8.2 (5) | |
| Mode of delivery | 0.98 | ||
| Vaginal delivery | 83.7 (587) | 83.6 (51) | |
| Cesarean section | 16.3 (114) | 16.4 (10) | |
| Gestational age (weeks) | 39 (38, 40) | 39 (38, 39.5) | 0.23 |
| K6 score (First trimester) | 2.0 (0.0, 6.0) | 4.0 (1.0, 7.0) | 0.02* |
| K6 score (Second/third trimester) | 2.0 (0.0, 4.0) | 3.0 (1.0, 5.0) | 0.14 |
| K6 score (1 year after delivery) | 1.0 (0.0, 4.0) | 2.0 (0.5, 5.0) | 0.06 |
| Edinburgh Postnatal Depression Scale score (1 month after delivery) | 5.0 (3.0, 7.0) | 4.0 (2.0, 8.0) | 0.62 |
| Edinburgh Postnatal Depression Scale score (6 months after delivery) | 4.0 (2.0, 6.0) | 4.0 (2.0, 6.0) | 0.66 |
| The Japanese version of the Mother-to-infant Bonding Scale (at 1 year postpartum) | 1.0 (0.0, 2.0) | 1.0 (0.0, 3.0) | 0.76 |
| Current employment status (1 year after delivery) |
0.60 | ||
| Employed | 15.6 (108) | 13.1 (8) | |
| Unemployed | 84.4 (583) | 86.9 (53) |
Data are median (interquartile range) or % (n). Pats history other than irregular menstruation, pregnancy complications other than diabetes, and obstetric complications other than gestational diabetes are listed in Supplemental Table 1. Data were missing on age at first childbirth (n = 56), parity (n = 9), highest level of maternal education (n = 2), highest level of partner's education (n = 5), annual household income during pregnancy (n = 36), age at first menstrual period (n = 26), infertility treatment (n = 1), history of taking oral contraceptives (n = 8), past history irregular menstruation (n = 7), K6 score (First trimester) (n = 13), K6 score (Second/third trimester) (n = 7), K6 score (1 year after delivery) (n = 11), Edinburgh Postnatal Depression Scale score (1 month after delivery) (n = 5), Edinburgh Postnatal Depression Scale score (6 months after delivery) (n = 14), The Japanese version of the Mother-to-infant Bonding Scale (at 1 year postpartum) (n = 15), Current employment status (n = 10). Distribution normality was confirmed using the Shapiro–Wilk test. Calculated using the Mann–Whitney U test or Chi-squared test (2 × 2 groups). *P < 0.05, **P < 0.01.
Table 4. Postpartum childcare environment according to the weeks of resuming postpartum menstruation.
| < 72 weeks n = 701 |
≥ 72 weeks n = 61 |
P | ||
| Level of satisfaction with the husband’s participation in childcare | Satisfaction | 80.3 (521) | 66.7 (40) | 0.01* |
| Dissatisfaction | 19.7 (128) | 33.3 (20) | ||
| Mother’s frequency of playing with the child | ≥ 5 times/week | 97.1 (672) | 100 (61) | 0.40 |
| < 5 times/week | 2.9 (20) | 0.0 (0) | ||
| Frequency of reading to the child | ≥ 5 times/week | 34.0 (236) | 29.5 (18) | 0.48 |
| < 5 times/week | 66.0 (458) | 70.5 (43) | ||
| Frequency of taking the child out of the house (to somewhere other than the childcare facility) | ≥ 5 times/week | 48.8 (339) | 49.2 (30) | 1.00 |
| < 5 times/week | 51.2 (355) | 50.8 (31) | ||
| Frequency of meeting other parents with the same-aged children | ≥ 5 times/week | 6.8 (47) | 5.0 (3) | 0.79 |
| < 5 times/week | 93.2 (647) | 95.0 (57) | ||
| Frequency of talking about the child with husband | ≥ 5 times/week | 80.4 (553) | 65.6 (40) | 0.01* |
| < 5 times/week | 19.6 (135) | 34.4 (21) | ||
| Availability of a caretaker other than the child’s mother | No | 3.9 (25) | 3.4 (2) | 1.00 |
| Yes | 96.1 (624) | 96.6 (57) | ||
| Availability of those whom the mother can seek advice about childcare | No | 1.1 (7) | 1.7 (1) | 0.50 |
| Yes | 98.9 (634) | 98.3 (57) | ||
| Breastfeeding | ||||
| At 6 months after delivery | No | 8.8 (58) | 0.0 (0) | 0.01* |
| Yes | 91.2 (600) | 100 (58) | ||
| At 1 year after delivery | No | 27.7 (192) | 3.3 (2) | < 0.01** |
| Yes | 72.3 (500) | 96.7 (59) | ||
| At 18 months after delivery | No | 81.1 (553) | 26.7 (16) | < 0.01** |
| Yes | 18.9 (129) | 73.3 (44) |
Data are median (interquartile range) or % (n). Data were missing on level of satisfaction with the husband's participation in childcare (n = 53), mother's frequency of playing with the child (n = 9), frequency of reading to the child (n = 7), frequency of taking the child out of the house (to somewhere other than the childcare facility) (n = 7), frequency of meeting other parents with the same-aged children (n = 8), frequency of talking about the child with husband (n = 13), availability of a caretaker other than the child's mother (n = 54), availability of those whom the mother can seek advice about childcare (n = 63), breastfeeding at 6 months after delivery (n = 46), breastfeeding at 1 year after delivery (n = 9), breastfeeding at 18 months after delivery (n = 20). Distribution normality was confirmed using the Shapiro–Wilk test. Calculated using the Mann–Whitney U test or Chi-squared test (2 × 2 groups).
*P < 0.05, **P < 0.01.
Table 5. Information on neonates according to the weeks of resuming postpartum menstruation.
| < 72 weeks n = 701 |
≥ 72 weeks n = 61 |
P | ||
| Single or multiple birth | single birth | 99.3 (696) | 100 (61) | 1.00 |
| multiple birth | 0.7 (5) | 0.0 (0) | ||
| Gender | male | 49.4 (346) | 50.8 (31) | 0.83 |
| female | 50.6 (355) | 49.2 (30) | ||
| Apgar score | At 1 minute after birth | 9.0 (8.0, 9.0) | 9.0 (8.0, 9.0) | 0.78 |
| At 5 minutes after birth | 10.0 (9.0, 10.0) | 10.0 (9.0, 10.0) | 0.19 | |
| pH value of umbilical arterial blood | 7.3 (7.3, 7.4) | 7.3 (7.3, 7.3) | 0.07 | |
| Birth weight (g) | ≥ 2500g < 2500g |
92.4 (648) 7.6 (53) |
90.2 (55) 9.8 (6) |
0.46 |
| Physical anomalies up to 1 month after birth | No | 78.7 (546) | 72.9 (43) | 0.30 |
| Yes | 21.3 (148) | 27.1 (16) | ||
| Disease diagnosed before 1 year of age | No | 75.1 (522) | 75.4 (46) | 0.96 |
| Yes | 24.9 (173) | 24.6 (15) | ||
Data are median (interquartile range) or % (n). Data were missing on apgar score (n = 6), pH value of umbilical arterial blood (n = 56), physical anomalies up to 1 month after birth (n = 9), disease diagnosed before 1 year of age (n = 6). Distribution normality was confirmed using the Shapiro–Wilk test. Calculated using the Mann–Whitney U test or Chi-squared test (2 × 2 groups).
Table 3 presents a comparison of category 1 (sociodemographic and clinical characteristics of participants) data. Significant differences were observed between the two groups in the following 8 variables: age at delivery, age at first childbirth, parity, infertility treatment, history of taking oral contraceptives, past history of irregular menstruation, gestational diabetes as an obstetric complication, and K6 score in the first trimester. There were no significant differences in the history other than irregular menstruation and obstetric complications other than gestational diabetes (Supplemental Table 1). Furthermore, there were no significant differences in any of the pregnancy complications (Table 3 and Supplemental Table 1).
Table 4 presents a comparison of category 2 (postpartum childcare environment) data. Significant differences were observed between the two groups in the following three variables: level of satisfaction with the husband’s participation in childcare at 1 year postpartum, frequency of talking about the child with the husband, and breastfeeding (at 6 months, 1 year, and 18 months after delivery).
Table 5 presents a comparison of category 3 (information on neonates) data. No significant differences were observed between the two groups in this category.
Table 6 shows the results of multivariate logistic regression analysis conducted to determine the factors related to postpartum menstrual resumption delay, with age at delivery, past history of irregular menstruation, infertility treatment, history of taking oral contraceptives, breastfeeding at 18 months after delivery, and level of satisfaction with the husband’s participation in childcare as independent variables. No multicollinearity was identified for the selected covariates. We used the Hostmer–Lemeshow test to evaluate the model’s goodness of fit. Significant relationships were observed between postpartum menstrual resumption delay and the following five items: age at delivery (OR 1.122, 95% CI 1.043–1.207), past history of irregular menstruation (OR 2.927, 95% CI 1.349–6.349), history of taking oral contraceptives (OR 2.351, 95% CI 1.062–5.204), breastfeeding at 18 months after delivery (OR 10.346, 95% CI 5.394–19.845), and the level of satisfaction with the husband’s participation in childcare (dissatisfaction: OR 2.047, 95% CI 1.031–4.064 based on satisfaction).
Table 6. Logistic regression analysis of factors associated with delay in resumption of postpartum menstruation.
| Variables | Odds ratio | 95% CI | P |
| Age at delivery (years) | 1.122 | 1.043–1.207 | < 0.01** |
| Past history of irregular menstruation | 1.349–6.349 | < 0.01** | |
| No (reference) | 1.000 | ||
| Yes | 2.927 | ||
| Infertility treatment | 0.688–6.470 | 0.19 | |
| No (reference) | 1.000 | ||
| Yes | 2.110 | ||
| History of taking oral contraceptives | 1.062–5.204 | 0.04* | |
| No (reference) | 1.000 | ||
| Yes | 2.351 | ||
| Breastfeeding at 18 months after delivery | 5.394–19.845 | < 0.01** | |
| No (reference) | 1.000 | ||
| Yes | 10.346 | ||
| Level of satisfaction with the husband’s participation in childcare | 1.031–4.064 | 0.04* | |
| Satisfaction (reference) | 1.000 | ||
| Dissatisfaction | 2.047 |
Cases used in the analysis: n = 685, cases excluded due to missing data: n = 77. *P < 0.05, **P < 0.01. CI, confidence interval.
DISCUSSION
Based on the report,1 menstruation has been considered to return in all women in the first 18 months postpartum regardless of their breastfeeding status. However, as shown in Fig. 1 and 2, 8.0% of Japanese women had not resumed menstruation even after 72 weeks postpartum. Although the race of the participants in the study by Campbell and Gray1 and that in our study are different, the delay was obvious in the comparison. Furthermore, a study in Sri Lanka with longitudinal follow-up reported that almost 100% of women resumed their menstruation by 60 weeks postpartum,34 irrespective of maternal nutritional status. It is obvious that our results indicate delayed menstrual resumption. Although previous studies from Japan have reported rates ranging from 21.2% to 38.8% at 6 months postpartum,6,7,8 no study has followed up Japanese women for a longer time period than our study. Thus, to the best our knowledge, there is no long-term study that clearly describes postpartum menstrual resumption delay in Japanese women, nor are there any large-scale surveys similar to the present one. Therefore, we examined the factors associated with postpartum menstrual resumption delay, including those previously identified.
In our study, the age at delivery of the delayed group was significantly higher than that of the normal group (34 years). The risk of postpartum menstrual resumption delay increased approximately 1.1–fold with every 1 year increase in the age at delivery. Advanced age significantly reduces ovarian function and lowers the ability to become pregnant.19 Age at first childbirth was significantly higher in the delayed group in the univariate analysis. Eleje et al.11 cited younger age as a factor in the early resumption of menstruation, supporting our results of a higher age at delivery in the delayed group. In Japan, while the mean age at the time of getting married and that at the time of childbirth continue to increase, this has slowed recently.35 Age at delivery can be considered as a factor for delayed resumption of menstruation leading to secondary infertility; hence, government’s policy against late marriages and childbirth is important.
Many textbooks and prior studies have described the effect of breastfeeding on postpartum amenorrhea.2, 9, 10 In this study, the breastfeeding rate was significantly higher in the delayed group at 6 months, 1 year, and 18 months after delivery. At 18 months postpartum, the odds ratio of breastfeeding women being in the delayed group was 10.3–fold that of non-breastfeeding women. In other words, as in previous studies, breastfeeding is strongly associated with postpartum menstrual resumption delay. Ishihara et al.6 demonstrated that among Japanese women, those who resumed menstruation at 6 months postpartum had a lower rate of breastfeeding than those who did not. This result is consistent with our study and the previous ones from other countries. Bai et al.36 reported that postpartum employment affects breastfeeding continuity. However, in this study, as shown in Table 4, breastfeeding had a greater effect on postpartum menstrual resumption delay than employment. For these reasons, it is imperative to adjust the timing of weaning in family planning for couples wishing to have next baby. A limitation of this study is that despite it being reported that breastfeeding for ≥ 15 min seven times a day delays ovulation resumption,1 it is unclear whether our study participants performed breastfeeding at 18 months under such conditions. Furthermore, the correlation between night-time breastfeeding and breastfeeding-related amenorrhea reported by the World Health Organization37 was not surveyed in our study. Furthermore, the detailed effects of prolactin could not be determined.
A history of taking oral contraceptives is associated with postpartum menstrual resumption delay. Oral contraceptives resemble ovarian hormones and inhibit the secretion of gonadotropin-releasing hormone from the hypothalamus, pituitary hormone secretion, and ovulation. It has been reported that, although not permanent, there is a concern that after stopping oral contraceptives, the ability to become pregnant is temporarily reduced.20 However, most previous studies have demonstrated that the ability to become pregnant is not associated with the use of oral contraceptives and that ovulatory function is restored after discontinuing their use.18, 21,22,23,24 Hassan et al.23 reported a cumulative pregnancy rate of 90% within 12 months of discontinuing oral contraceptives. Hull et al.24 also noted that oral contraceptives do not cause ovarian dysfunction. In our study, the duration of oral contraceptive usage, type of agent used, ovarian function at the start of oral contraceptive usage, and reasons for oral contraceptive usage were not evaluated, which constitute the limitations of this study. In the future, it is necessary to consider what happens after the non-ovulatory period with oral contraceptives and the physiological non-ovulatory (ovulation suppression) period after pregnancy and childbirth have passed. In other words, we believe it is worth considering whether or not the level of ovulation will return to the level prior to the use of oral contraceptives. Furthermore, our univariate analysis revealed a significantly high proportion of women in the delayed group with a history of irregular menstruation and infertility treatment. Some participants in this study had underlying premature ovarian failure; we believe that ovarian function might have developed after some time had elapsed, such as from oral contraceptive usage and pregnancy/childbirth. Unfortunately, the causal relationship between taking oral contraceptives and postpartum menstrual resumption delay is unclear in our study.
K6 scores are used to screen for depression and anxiety, and in this study, these scores were significantly higher in the first trimester for the delayed group. However, there was no significant difference in K6 scores in the second and third trimesters and at 1 year postpartum. We believe that the first trimester has the least impact on menstruation and that it is highly likely that the significance was due to incidental factors.
In the univariate analysis, significant differences were found in gestational diabetes as an obstetric complication. However, the number of patients with the obstetric complication of gestational diabetes was too small to be included in the multivariate analysis: four in the normal group and five in the delayed group. In addition, there was no significant difference in diabetes as a pregnancy complication, which is a more long-term abnormality in glucose metabolism than gestational diabetes. There are no existing reports stating that abnormality in glucose metabolism affects ovulation; hence, we considered this result incidental and not clinically significant.
There was no significant relationship between information on neonates and postpartum menstrual resumption delay, and infant status at birth and physical complications were not associated with postpartum menstrual resumption delay. However, Aryal et al.12 reported that child death is a factor in the early resumption of menstruation. Because we excluded child death in our study, we have not been able to investigate the impact of child death. The possibility that postpartum menstrual resumption delay is associated with acquired diseases, such as developmental disorders that can increase maternal stress due to childcare difficulties, cannot be ruled out.
Furthermore, the most novel finding of this study was that participants who felt “unsatisfied” with their husband’s participation in childcare at 1 year postpartum had an approximately 2.0–fold higher risk of postpartum menstrual resumption delay. It has been reported that compared with countries overseas, Japanese fathers spend little time on housework, including childcare, at 114 min/week, in contrast to 448 min/week by mothers.38 In previous Japanese research, it has been reported that the higher the frequency of childcare by the father, the lower the maternal psychological distress.25 Lin et al.26 also noted that paternal participation in childcare is related to maternal mental health. Therefore, when the wife feels unsatisfied with her husband’s participation in childcare, it is possible that the psychological stress causes hypothalamic inhibition of ovulation and postpartum menstrual resumption delay. The husband’s cooperation with childcare is to be established in the marital relationship. According to Table 4, there was no significant difference in the variables that showed the relationship with children, but there was a significant difference in the variables that showed the relationship between husband and wife. This is an interesting result. Unsatisfactory participation of the husband in child rearing and talking with the husband about the child less than five times a week put the woman at risk of postpartum menstrual resumption delay. Our study suggests the importance of increasing the intimacy between the couple and the husband’s participation in childcare. To the best of our knowledge, no reports to date have described the childcare environment and its relationship with the psychological aspect of the mother caused by the husband–wife relationship with respect to the postpartum menstrual resumption delay. This manuscript is the first to report such factors. However, this survey did not obtain the specifics of their dissatisfaction. A more detailed survey would help us find specific solutions for husbands’ participation in childcare.
Determining the factors associated with postpartum menstrual resumption delay will help in the early detection of secondary infertility and is of great benefit for family planning. This provides important information to guide married couples who hope to become pregnant with a next child to consult an expert. In addition, this study supports Reproductive Health and Rights. Our results may lead to healthier women and their families choosing to have children. It will also help the midwives who provide them with evidence-based care. The results provide an important basis for further research.
SUPPLEMENTARY MATERIALS
Acknowledgments
Acknowledgments: We would like to express our gratitude to everyone in the Kamiina district in Nagano prefecture for enrolling in the JECS at Shinshu University and for their cooperation in completing the questionnaire.
The Japan Environment and Children’s Study was funded by the Ministry of the Environment, Japan. This study was supported in part by grants from JSPS KAKENHI (Grants Number 25460794) and Management Expense Grant. The conclusions of this article are solely the responsibility of the authors and do not represent the official views of the above government.
Footnotes
The authors declare no conflict of interest.
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